AssigneeChanged event

Fired when a change's assignee has been changed or removed:

  type: "assignee-changed"
  change: The updated change
  changer: The user that issued the assignment
  oldAssignee: Assignee before it was changed.
  eventCreatedOn: Timestamp describing when this event was created.

Change-Id: I5bf1f3f71b84afda82ecb1058a9cbf04185474f3

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/AssigneeChangedListener.java

@@ -0,0 +1,29 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.events;
+
+import com.google.gerrit.common.Nullable;
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+import com.google.gerrit.extensions.common.AccountInfo;
+
+/** Notified whenever a change assignee is changed. */
+@ExtensionPoint
+public interface AssigneeChangedListener {
+  interface Event extends ChangeEvent {
+    @Nullable AccountInfo getOldAssignee();
+  }
+
+  void onAssigneeChanged(Event event);
+}